Easily confused with other
Water-Milfoils, of which it is a member, until it is understood that this is the only Water-Milfoil that is partly above the water and not wholly submerged.
This is another problematic plant in that it quickly invades any still freshwater where it has been discarded by aquatic gardeners, to the detriment and ultimately entire exclusion of all other aquatic plants and fish in the pond. It has spread very quickly from being virtually un-known in Britain two decades ago. If it should start growing in any lake, pond, canal, reservoirs, flooded quarries or other still waters it should be reported so that the infestation of this foreign invader can be dealt with by the authorities.
The upper leaves can turn a brilliant crimson-red which contrasts well with lower still-green leaves making for a spectacular show en-masse.
A dioecious plant, with male and female flowers on separate plants. However, almost all plants outside of South America are female, including the ones pictured above, and have tiny white flowers only 2mm across with up to 6 in a whorl around the stem in the axils of the leaves. Like the leaves, the flowers are in whorls and emerge in the axils of the emergent stems. The male plants have no flowers. The plant therefore does not reproduce sexually outside of South America, and reproduces only via vegetative propagation or by root division or from plant fragments. No source seems to indicate how many petals the (female) flowers have.
It is tolerant to frost and present throughout the year. Becoming commoner in Southern Britain.